1. Field of the Invention
This invention relates to a pressure control valve unit for a hydraulic brake device for vehicles equipped with an antilock brake system (ABS) or a traction control system (TCS).
2. Description of the Related Art
A hydraulic brake device that can electronically control the brake pressure to prevent the wheels from locking during a sudden braking operation or a sudden acceleration is publicly known. It is known that the alternate opening and closing of the solenoid-type inlet and outlet valves in that type of device is accompanied by an abnormal noise, referred to as noise, vibration, and harshness (NVH), and vibration.
Two methods have been proposed as means of eliminating this NVH; the placement of a pressure-actuated switching valve on the downstream side of the inlet valve in the main brake line, or the placement of an independent pressure-actuated switching valve on the upstream side of the inlet valve. However, these methods have the following problems:
1. Placement of a pressure-actuated switching valve on the downstream side of the inlet valve should reduce the NVH, but in some cases there is a prominent high frequency noise.
2. Placement of an independent pressure-actuated switching valve on the upstream side of the inlet valve does reduce the NVH to a certain degree, but a sudden abnormal noise and vibration is generated. Moreover, the introduction of an independent switching valve makes the layout design more complex, and increases the cost.
3. Although experiments have been conducted to incorporate the switching valve inside another solenoid valve, the problems of size and cost have yet to be resolved, and such a design is not yet in practical use.